ABSTRACT
This chapter focuses on the way an industrial controller operates to see how the mathematics of the control actions, developed in the previous chapter, are put to practical use. It shows by relatively simple mathematics, some of the relationships between the control actions and the process to which they have been applied, the objective being to understand what is occurring in the process stream under the action of the controller. The chapter explains the well-known phenomenon of an ever-increasing controller output observed when a controller is either deliberately or inadvertently disconnected from the controlled device. The comparator receives the scaled signal and makes a comparison between it and the desired value. The error signal thus generated is passed on to the control unit. In pneumatic instruments this will consist of a number of bellows operating on a common beam, which achieves balance when the measurement is equal to the setpoint.
